If you extract the grain (e.g. by subtracting a blurred version of the image), the result contains mostly noise, not meaningful scene information outside of some variation according to image brightness. Film grain is random, so the extracted "grain layer" doesn’t encode original image detail but film grain itself encodes relative lightness in its _density_.
Mostly noise but not just noise. There is a reason why quality de-noising is a hard problem and the lost information becomes more apparent for videos where our brain is better able to extract it from the original noise.
